A packaging material for foodstuffs in forms of film, sheet, bag, bottle, etc.; a packing material for containers used for a seal between a beverage bottle and a crown cap, a seal between a medical drug bottle and a lid, etc.; a material for medical infusion bags; a tube material for tires; a cushion material for shoes; and the like are required to have gas barrier properties and flexibility.
For example, for a packaging material for food and beverages, a laminate comprising a flexible resin layer and a gas barrier layer formed from an ethylene-vinyl alcohol copolymer or from a polyamide is widely used since flexibility and a high degree of gas barrier properties are required. For a packing material for containers, NR (natural rubber) and/or IIR (butyl rubber) are generally used since flexibility and gas barrier properties are required. For a material for medical infusion bags, vinyl chloride is generally used since flexibility and gas barrier properties are required. For a tube material for tires, IIR is generally used since a high degree of gas barrier properties and flexibility are required.
However, in order for the aforementioned molded articles obtained by employing NR and/or IIR to develop flexibility, a complex vulcanizing step is required after molding. In the packaging material for food and beverages, because of being superior in gas barrier properties but having insufficient flexibility, the ethylene-vinyl alcohol copolymer or polyamide is used in a form of a laminate with a flexible resin layer for compensating the insufficiency, leading to restriction and complexity in molding processing. NR does not have a very high degree of gas barrier properties, and therefore is disadvantageous for use as a packing material for containers etc., in terms of a long term storability of a content. Meanwhile, since vinyl chloride contains a large amount of chlorine atoms, molded articles such as medical infusion bags formed from vinyl chloride are feared to have a negative impact on environment due to gas such as hydrogen chloride generated upon incineration disposal.
In this regard, as a polymeric material that is superior in flexibility and not requiring a vulcanizing step, thermoplastic elastomers such as SEBS (a styrene-ethylene/butylene-styrene triblock copolymer) and SEPS (a styrene-ethylene/propylene-styrene triblock copolymer) have been proposed. However, gas barrier properties of these thermoplastic elastomers may be insufficient depending on intended use.
In this regard, as a resin composition that gives a molded product usable for a packaging material for food and beverages, a packing material for containers, a material for medical infusion bags, a tube material for tires, etc., resin compositions each containing: a block copolymer having a polymer block that includes a vinyl aromatic monomer unit, and a polymer block that includes an isobutylene unit; and an ethylene-vinyl alcohol copolymer have been proposed (refer to Patent Document 1 and Patent Document 2). The resin composition disclosed in Patent Document 1 is characterized in that the ethylene-vinyl alcohol copolymer is present as a dispersed particle phase in a matrix phase of the block copolymer. Whereas, the resin composition disclosed in Patent Document 2 is characterized in that the block copolymer is present as a dispersed particle phase in a matrix phase of the ethylene-vinyl alcohol copolymer.
The resin composition disclosed in Patent Document 1 is reportedly capable of providing molded articles that are superior not only in flexibility, but also in gas barrier properties. Meanwhile, the resin composition disclosed in Patent Document 2 reportedly results in an improvement of flexibility of molded articles while leveraging gas barrier properties of the ethylene-vinyl alcohol copolymer.
However, the present inventors have investigated to find that a balance between gas barrier properties and flexibility of the molded articles formed from the aforementioned resin compositions can be further improved.
Among the aforementioned resin compositions, the resin composition containing an ethylene-vinyl alcohol copolymer as a matrix phase is, as appropriate according to intended use, generally used as a gas barrier layer in a multilayer structure having an appropriate polyolefin layer (e.g., a polyethylene layer, etc.). However, since an ethylene-vinyl alcohol copolymer has poor adhesiveness to a polyolefin, an adhesive layer or an adhesive is required between layers in order to form a multilayer structure with an ethylene-vinyl alcohol copolymer, leading to restriction and complexity in molding processing.